1. Field of the Invention
The present invention relates generally to the field of ball valves and more particularly concerns trunnion type ball valves having valve seats that are moveable with the valve and establish sealing engagement with a valve ball member that is supported for rotational movement between open and closed positions. More particularly the present invention concerns trunnion type ball valves having seat members that are laterally moveable within seat recesses for self-centering and optimum sealing engagement with the spherical sealing surface of the trunnion mounted valve ball member.
2. Description of the Prior Art
The two basic types of ball valves, floating ball valves and trunnion ball valves, are each designed for opening and closing movement with a quarter turn of rotary movement of a valve ball. Floating ball valves employ valve seats that may be fixed or moveable within a valve chamber and employ a valve ball member that has the capability of being moved either mechanically or by pressure response so as to seek an optimum sealing relation with respect to the downstream seat assembly of the valve. Typically, floating ball valves can be of relatively low cost manufacture as compared with other ball valves and are typically quite satisfactory for low to medium pressure service. One of the disadvantages of floating ball valves is the unusually significant seat wear that often occurs as the result of pressure responsive valve ball force against the downstream seat of the valve. Frequent maintenance for seat replacement may be needed, thus requiring a flow line to be shut down for maintenance more frequently than is desired. When elevated pressure conditions are encountered, many floating ball valves can require high torque forces for opening and closing movement and many experience significant seat wear, particularly downstream seat wear, since greater pressure responsive force of the valve ball member is often applied to the downstream seat.
Trunnion ball valves typically have a valve body within which a valve ball member is mounted for rotation by trunnions, trunnion supports and trunnion bearings. A trunnion mounting system typically permits the valve ball to be rotated to its open and closed positions and prevents the valve ball from being subjected to pressure responsive movement within the valve body. Trunnion ball valves are typically of more expensive manufacture as compared with floating ball valves; however the more expensive manufacturing cost is typically offset by desirable features that are known in these types of ball valves. Typically trunnion type ball valves are capable of operating at higher pressure conditions as compared with floating ball valves. Since the valve ball member is supported for rotation within the valve body and is prevented from downstream movement by differential pressure, there is no tendency for a condition of high operating torque to be experienced unless some other factor becomes evident. The valve seats of trunnion type ball valves are not normally subject to excessive wear during operation. One or more seats may be mechanically energized such as by means of seat springs and may be energized responsive to differential pressure to ensure that efficient sealing will occur at all pressure conditions, without experiencing unusual seat wear when operating pressure becomes elevated. And, since the valve ball is stabilized within the valve body by the trunnion mounting system, there is no tendency for the sealing surface of the valve ball to become unusually worn or eroded during normal operation.